CD52 is a small molecule that is located on the cell surface of some inflammatory cells (mainly lymphocytes, natural killer cells, and monocytes). CD52 is not expressed on hematopoietic progenitors, erythrocytes, or platelets. Binding to the CD52 antigen of inflammatory cells causes complement- mediated cell lysis. Furthermore, it is found within the male genital tract and is present on the surface of mature sperm cells. CD52 plays complicated roles on T-cell activation via multiple pathways. It can modulate T-cell activation either by its intracellular signal pathways or by the interaction of soluble CD52 and Siglec-10 expressing on T cells. CD52 acts as costimulatory molecules to activate effectors and regulatory T cells.